In an analog signal path which comprises amplifier stages and filter stages, it is desirable to carry out as much as possible of the required amplification in a first amplifier stage in order to optimize the signal-to-noise ratio. This also applies in particular to a reception line of an xDSL (“digital subscriber line”) device, or an xDSL modem. In the event of large amplification, however, care must be taken to ensure that the elements following this first stage in the analog signal path are not overloaded. This may particularly be the case for integrated circuits with a low current supply. It may be therefore necessary to amplify the signal, then filter it or process it in another way, and then amplify it again. This, however, is not optimal for the signal-to-noise ratio.
A particular problem occurs here in reception devices, especially of ADSL central offices. The transmission power spectrum p is represented as a function of the frequency fs for ADSL communication by way of example in FIG. 3. Between frequencies f1 and f2, there is a first frequency band 5 for the data transmission from a terminal or a subscriber connection to the central office (the so-called upstream direction), above which there is a second frequency band 6 between the frequency f2 and a frequency f3 for the data transmission from the central office to the subscriber connection (the so-called downstream direction). The second frequency band 6 here covers a larger frequency range than the first frequency band 5, since more data normally have to be transmitted from the central office to the terminal than in the opposite direction. These frequencies f1, f2 and f3 may depend on the ADSL standard being used, and on whether the ADSL transmission is taking place on a conventional telephone line (POTS, “plain old telephone system”) or on an ISDN line. For ADSL combined with ISDN, for example, f1 is 138 kHz, f2 is 276 kHz and f3 is 1104 kHz.
Owing to the frequency distribution as represented in FIG. 3, there is a dominant echo of the transmission signal in a reception signal path of the central office, that is to say a signal lying in the second frequency band 6. This signal thus makes up a large part of the energy in the reception signal and of a peak voltage. This means that the echo signal crucially determines how great the amplification can be in a first stage of the reception signal path, in order to avoid overloading a subsequent analog-digital converter. This leads to the aforementioned problem regarding the signal-to-noise ratio.
There are two known ways of attenuating this effect and this dominance of the echo signal:    1. A hybrid circuit for two-wire/four-wire conversion, that is to say for separating a signal present in a transmission line into the reception signal and a transmission signal, may be optimised so that as little echo signal as possible enters the reception path. This option is limited both on the technical side and on the cost side.    2. Lowpass filtering may be carried out before the amplification, in order to filter out the echo signal as much as possible. Such a reception path is schematically represented in FIG. 4.
A reception signal a is in this case delivered to a lowpass filter 7, which is intended to filter out the echo signal that the reception signal contains. A signal e obtained in this way is then delivered to an amplifier unit 8 which amplifies the signal e according to requirements, in particular according to its signal strength. The resulting signal f may be filtered again by a filter unit 9, and the signal g provided at the output of the filter unit 9 is then converted by an analog-digital converter 3 into a digital signal h for further processing.
The lowpass filter 7 must have a relatively low cutoff frequency here, close to f2 in FIG. 3, so that it always filters out as much of the echo signal as possible. This requires relatively large resistances and capacitances, the effect of which is that the lowpass filter 7 on the one hand requires a large chip area and, on the other hand, it is difficult to optimise in terms of the signal-to-noise ratio because the cutoff frequency is set.